Internet Protocols: Understanding Multicasting and IGMP, Slides of Data Communication Systems and Computer Networks

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Chapter 19

Internetwork Protocols

Example

Config

True Multicast

• Determine least cost path to each network that

has host in group

—Gives spanning tree configuration containing networks with group members

• Transmit single packet along spanning tree

• Routers replicate packets at branch points of

spanning tree

• 8 packets required

Multicast Example

Requirements for

Multicasting (2)

• Mechanism required for hosts to join and leave

multicast group

• Routers must exchange info

—Which networks include members of given group —Sufficient info to work out shortest path to each network —Routing algorithm to work out shortest path —Routers must determine routing paths based on source and destination addresses

Spanning Tree from Router C to

Multicast Group

Principle Operations

• Hosts send messages to routers to subscribe to

and unsubscribe from multicast group

—Group defined by multicast address

• Routers check which multicast groups of interest

to which hosts

• IGMP currently version 3

• IGMPv

—Hosts could join group —Routers used timer to unsubscribe members

Operation of IGMPv1 & v

• Receivers have to subscribe to groups

• Sources do not have to subscribe to groups

• Any host can send traffic to any multicast group

• Problems:

—Spamming of multicast groups —Even if application level filters drop unwanted packets, they consume valuable resources —Establishment of distribution trees is problematic —Location of sources is not known —Finding globally unique multicast addresses difficult

IGMP Message Formats

Membership Query

Membership Query

• Sent by multicast router

• General query

—Which groups have members on attached network

• Group-specific query

—Does group have members on an attached network

• Group-and-source specific query

—Do attached device want packets sent to specified multicast address —From any of specified list of sources

Membership Query Fields (2)

• QRV (querier's robustness variable) — RV value used by sender of query — Routers adopt value from most recently received query — Unless RV was zero, when default or statically configured value used — RV dictates number of retransmissions to assure report not missed
• QQIC (querier's querier interval code) — QI value used by querier — Timer for sending multiple queries — Routers not current querier adopt most recently received QI — Unless QI was zero, when default QI value used
• Number of Sources
• Source addresses — One 32 bit unicast address for each source

IGMP Message Formats

Membership Report

IGMP Message Formats

Group Record

Group Record

• Record Type

—See later

• Aux Data Length

—In 32-bit words

• Number of Sources

• Multicast Address

• Source Addresses

—One 32-bit unicast address per source

• Auxiliary Data

—Currently, no auxiliary data values defined